The present invention relates generally to improved methods and apparatus for broadband tunable generation or amplification of coherent electromagnetic radiation at millimeter, sub-millimeter and infra-red wavelengths utilizing a non-relativistic electron beam for terrestrial, space and air-born communication, radars, semiconductor manufacturing, medical and other applications. More particularly, an electron device consisting of an electron gun for producing a non-relativistic electron beam, and techniques that are described below for creating uniform axial magnetic field B0 and periodic transverse magnetic field Bw within a device interaction region so that said electron beam moves along a helical trajectory with the transverse velocity vxe2x8axa5 and the longitudinal velocity v∥ satisfying the following relationship             v      ⊥      2        ≥                  c        2            ⁢              (                                            Λ              w                                      Λ              0                                -          1                )              ,
where c is the speed of light in vacuum, xcex9w is the spatial period of the helical electron trajectory in the combined field (it is also the spatial period of the transverse magnetic field), xcex90 is the spatial period of the cyclotron revolution of the electron in the axial guide field B0,             Λ      0        =                  v        ||            ⁢                        2          ⁢          π          ⁢                      xe2x80x83                    ⁢          m          ⁢                      xe2x80x83                    ⁢          c                          e          ⁢                      xe2x80x83                    ⁢                      B            0                                ,
v∥ is the longitudinal electron velocity, e and m are the charge and mass of the electron, respectively.
Broadband tunable sources of electromagnetic radiation in millimeter, submillimeter and far-infrared bands are widely sought for a number of applications such as space and airborne communication, radars, medical applications, semiconductor manufacturing and others. Recently, broadband was added to a list of requirements to be met for a number of broadband-hungry digital wireless communication and Internet related applications. Although this region of the electromagnetic spectrum cannot be labeled as unreachable with traditional vacuum or quantum electronics devices, the existing devices have low efficiency, narrow bandwidth and are not tunable. The point is that this region of spectrum is situated in between regions well occupied by vacuum electron devices such as travelling wave tubes (TWT), backward wave oscillators (BWO), klystrons and magnetrons on the mm wavelength side and solid state quantum devices on the infrared and shorter wavelength side.
Traditional vacuum electron devices, such as traveling wave tubes (TWTs), use either a slow-wave structure with the period L≈v∥/f, where f is the device operating frequency, or in the case of the so-called gyro-devices, a high intensity axial magnetic field B0 such that electron cyclotron frequency             e      ⁢              xe2x80x83            ⁢              B        0                    2      ⁢      π      ⁢              xe2x80x83            ⁢      m      ⁢              xe2x80x83            ⁢      c        ≈  f
is close to the device operating frequency. For the frequencies above 300 GHz (wavelength of 1 mm or shorter), a slow-wave structure with a period less than 1 mm would be required. In addition to being not technologically feasible, in such small period slow wave structures, it is impossible to realize efficient interaction of an electromagnetic field with an electron beam. In the case of gyro-devices at frequencies above 300 GHz, an axial magnetic field stronger than 10 kGs would be required which cannot currently be met in a portable device. Thus, further advance of the traditional vacuum electronics into higher frequencies (shorter wavelengths) requires development of new principles.
On the other hand, solid-state quantum devices are not efficient in this region of the spectrum because the operating wavelength is too long for quantum effects to be significant.
Among known devices, only free electron lasers (FEL) are efficient in this region. The reason for this is probably the fact that the FEL utilizes principles of quantum electronics in medium such as an electron beam which is usual for classical vacuum electronics. Thus, the essential parts of both quantum and classical electronics are combined in this device. Unfortunately, for an FEL to operate in the submillimeter region, an electron beam with the energy of at least several MeVs is needed. Consequently, neither the dimensions nor price of such an FEL are suitable for most of the applications mentioned above.
The present invention further develops FEL principles leading to the creation of novel tunable vacuum electron devices able to generate and/or amplify electromagnetic radiation in the super-wide wavelength band ranging from millimeters to far-infrared (or above 30 GHz to approximately 30 THz). The physical mechanism of such devices is close to the mechanism of the wideband regime of long wavelength FEL operation. Because of the physical nature of the underlying instability, the FEL operating frequency in this regime is not determined by the electron beam energy and wiggler field period. It has been shown that a frequency region within which an amplification or generation of electromagnetic waves occurs spans from slightly below to far above the resonant FEL frequency. Therefore, in practical implementations, the frequency band is determined by the frequency characteristics of an FEL resonator and interaction region. The frequency band can be widely tuned without changing the electron beam energy and/or wiggler period. The operating frequency band is upper limited by the thermal spread of the electron beam. Although this regime cannot be understood without using the relativistic equation of motion and, consequently, was not discovered in the classical vacuum electronics, the regime itself does not rely upon relativism of an electron beam. Thus, a non-relativistic implementation of such regime is possible.
To this end, an innovative approach for developing a source of coherent electromagnetic radiation at frequencies 30 GHz and higher is provided. Unlike traditional vacuum electron devices, the device of the present invention does not utilize a strong axial magnetic field or slow-wave structure, because it does not rely on beam-wave synchronism. Instead, it uses a principle of parametric interaction of waves in an electron beam which is successfully realized in relativistic electronics, and in particular in free-electron lasers (FELs).
A conventional FEL configuration is based on an interaction of a fast electromagnetic mode of a waveguide (usually cylindrical) with an electron beam which, under presence of combined axial guide field and helical wiggler field, moves along a helical trajectory with the spatial period equal to the period of the wiggler field, xcex9w. A beating between the electromagnetic wave and the periodic transverse electron velocity produces a periodic longitudinal force which affects the longitudinal motion of the beam (this process is usually referred to as excitation of space charge waves of an electron beam). Modulation of beam velocity eventually results in modulation of beam density which, in its turn, creates an up-frequency shifted electron current that interacts with the initial electromagnetic wave. In other words, in an FEL, a high-frequency electromagnetic wave interacts with the space-charge waves of an electron beam and the interaction is possible because electron motion is periodic in the presence of an axial and a wiggler magnetic field as discussed further below. Because of the fact that space charge waves have phase velocity close to the longitudinal velocity of the electron beam, v∥, and the electromagnetic wave""s phase velocity is practically equal the speed of light, c, the interaction is synchronous and leads to an amplification or generation of the electromagnetic wave only within a narrow frequency band near the resonant frequency   f  ≈                    v        ||                              Λ          w                ⁢                  (                      1            -                                          v                ||                            /              c                                )                      .  
Apparently, the resonant frequency could be very high for a relativistic electron beam, when v∥xe2x86x92c. Unfortunately, a several MeV electron beam is required to produce millimeter wave radiation, and the operating wavelength could only be tuned at the expense of changing the electron beam energy or wiggler field period both of which are not currently realizable in any practical, portable application.
However, if a certain relation between the axial magnetic field and transverse magnetic field holds, the FEL operational frequency band greatly expands toward higher frequencies and, consequently, the operating frequency is no longer determined by the resonance formula above.
This regime features an interaction between different eigen modes of the electron beam, namely cyclotron waves and space-charge waves. Because the phase velocity of each of these waves does not depend on the frequency (in fact, it is close to the longitudinal beam velocity), a band of synchronism between waves becomes extremely broad and ranges from slightly below to far above the conventional FEL resonance frequency. The operating frequency band is determined by the electrodynamic characteristics of the device interaction region and device resonator. Thus, the electron device can be tuned within the above said band without changing the electron beam energy. The frequency band is up-limited by the thermal velocity spread in the electron beam. For a good quality electron beam, an estimated limiting frequency is on the order of 30 THz-100 THz.
Although this regime cannot be understood without using the relativistic equation of motion of an electron beam, it does not rely on the relativism of the longitudinal electron velocity. Among its other aspects, the present invention advantageously realizes this regime in a portable non-relativistic electron device.
These and other features, aspects and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken together with the accompanying drawings.